<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <title>demo</title>
  <script src="../vue.js"></script>
</head>
<style>
  #app div {
    width: 500px;
    height: 500px;
    background-color: orange;
  }
</style>

<body>
  <div id="app">
    <div :style="{backgroundColor: fullName}"></div>
    <input type="text" v-model.number="bga1">,
    <input type="text" v-model.number="bga2">,
    <input type="text" v-model.number="bga3"> =
    <input type="text" v-model='fullName'>
  </div>
  <script>
    const vm = new Vue({
      el: "#app",
      data: {
        bga1: '',
        bga2: '',
        bga3: '',
      },
      computed: {
        fullName() {

          return `rgb(${this.bga1},${this.bga2},${this.bga3})`
        }
      }

    })
  </script>
</body>

</html>